1. Field of the Invention
This invention relates in general to a Wavelength Division Multiplexing (WDM) technology used in a network access system, and more particularly to a hybrid analog/digital WDM technology used in a network access system.
2. Description of Related Art
The phenomenal growth of Internet and other broadband applications has generated more and more data transport demand in network communication industry. Fiber optical communication networks have been traditionally used in the local or wide area networks. Traditional Hybrid Fiber Coax (HFC) network is generally shown in FIG. 3 (see later for details). Usually, the digital and/or analog signals are first transformed into optical signals, and then the optical signals are transmitted via an optic fiber from a remote provider to a user end. As shown in FIG. 3, the signals (arrow-down) are split by a splitter into several branches (usually four branches in the existing CATV network). The optical signals in each fiber branch are then converted into electrical signals at an optical node. The converted electrical signals are next sent to various users via coax cables either via bus configuration (as shown) or star configuration (not shown). In this conventional hybrid fiber coax network, the digital service typically use 550-750 MHz for downstream (i.e. from a service provider to a user), assuming a coax amplifier allows bandwidth up to 750 MHz, and use 5-50 MHz for upstream (i.e. from a user to a service provider), which translates into about 800 Mbps (Mega bits per second) downstream and 160 Mbps upstream using 64 QAM for modulation. This data transport capacity can be typically shared by about 2000 users in one fiber serving area as illustrated in FIG. 3. However, with the high demand for bandwidth (i.e. the signal transport capacity), 800 Mbps is not enough at all, not to say the much slower upstream signal transport capacity.
Further, due to the fact that bandwidth provided in the upstream direction is in the lower band arrange, the signals can be readily interfered by a noisy lower band. Furthermore, if bandwidth itself is very narrow, it would significantly reduce the speed of signal transfer in an upstream transmission.
To understand further on the limitation and disadvantages of the existing network systems, the emphasis should be made to focus on the source and current solutions to the challenges faced by telecommunication industry. It can be seen that as more and more users start to use data networks, and as the usage evolves to include more and more bandwidth, intensive networking applications such as data browsing on the World Wide Web (WWW), Java applications, video-conferencing, etc., there emerges an acute need for very high-bandwidth transport network facilities, whose capabilities are much beyond those that current high-speed asynchronized transfer mode (ATM) networks can provide. The "network lag" in Internet creates more and more problems for users to access a World Wide Web server especially to display a picture. The demand for increasing the bandwidth in today's networks becomes a priority in every communication system. Realizing that the maximum rate at which an end-user (e.g. a work station or a gateway that interfaces with lower-speed subnetworks) can access the network is limited by electronic speed, the key in designing optical communication networks in order to exploit the fibers huge bandwidth is to introduce concurrency among multiple user transmissions into the network architectures and the protocols. In an optical communication network, this concurrency is often provided according to either wavelength or frequency, i.e., wavelength-division multiplexing (WDM). A local optical network that employs wavelength-division multiplexing is referred to as a wavelength-division multiple access (WDMA) network.
Wavelength-division multiplexing (WDM) is an approach that can exploit the huge opto-electronic bandwidth mismatch by requiring that each end-user's equipment operate only at electronic rate, but multiple WDM channels from different end-user's may be multiplexed on the same fiber. Under WDM, the optical transmission spectrum is carved up into a number of non-overlapping wavelength (or frequency) bands, with each wavelength supporting a single communication channel operating at whatever rate one desires. Accordingly, by allowing multiple WDM channels to co-exist on a single fiber, a user can tap into a huge fiber bandwidth, with the corresponding challenges being the design and development of appropriate network architectures, protocols, and algorithms.
The WDM network constructions are varied time-by-time according to the demand of the networks and the end users. One type of the WDM network's construction is a broadcast-end-select (local) optical WDM network. The local WDM optical network is often constructed by connecting network nodes via two-way fibers to an optical de/multiplexer. A node sends its transmission to the optical multiplexer on one available wavelength, using a laser which produces an optical information stream. The information streams from multiple sources are optically combined by the optical multiplexer, and the signal power of each stream is forwarded to all of the nodes on their receive fibers. A node's receiver, using an optical filter, can be tuned to only one of the wavelengths; hence, it can receive the information stream.
The second most popular WDM network construction is the wavelength-routed (wide-area) optical network. The wide-area optical WDM network includes a photonic switching fabric having active switches connected by fiber links to form an arbitrary physical topology. Each end user is connected to an active switch via a fiber link. The combination of an end user and its corresponding switch is referred to as a network node. Each node (i.e. access station or headend) is equipped with a set of transmitters and receivers, both of which can be wavelength tunable. A transmitter at a node sends data into the network and a receiver receives data from the network. One basic mechanism of communication in a wavelength-routed network is a light path. A light path is an all-optical communication channel between two nodes in the network. The intermediate nodes in the fiber path route the light path in the optical domain using their active switches. The end-nodes of the light path access the light path with transmitters and receivers that are tuned to the wavelength on which the light path operates. A fundamental requirement in the wavelength-routed optical network is that two or more light paths traversing the same fiber link must be on different wavelength channels so that they do not interfere with one another.
Current wide-area networks are designed as electronic networks with fiber links. However, these networks may not be able to take full advantage of the bandwidth provided by optical fibers, because electronic switching components may be incapable of switching the high volume of data which can be transmitted on the fiber links. The next generation of optical networks are being made using optical routers and switching elements to allow all-optical light paths to be set up from a source node to a destination node, thus bypassing electronic bottlenecks at intermediate switching nodes.
With more and more new multi-media services or applications are being used on a network, the cost-effective upgrading of the access network is a challenging task for both operators and vendors. There is a need for an improved hybrid analog-digital WDM access network to solve the network upgrading problem without rebuilding the existing network. In particular, the improved hybrid analog-digital WDM access network has to be compatible with both the existing hybrid fiber-coax (HFC) and switched digital video (SDV) infrastructures.
One solution to the above problems is to replace an entire network system by using high-bandwidth, high-throughput, new generation of optic fibers. This is extremely expensive.
Another solution is often referred to a Fiber To The Curb (FTTC). FTTC network architecture is generally shown in FIG. 4. Each optical network unit (ONU) (also called digital optical node) is connected to a network headend via a pair of fibers. To build a FTTC network, it requires deploying a new fiber plant to make sure that each of the ONUs connects to the network headend. It is also very expensive, especially in the metropolitan areas where the fiber ducts are running in/out and the right of way usually poses significant constrains. Further, for long haul carriers, power loss is very significant. Amplifiers are often used to maintain the quality of the signals. In FIG. 4, the increase of amplifiers almost becomes exponential. The cost associated with additional amplifiers could be very substantial. Accordingly, the proposed FTTC network is only limited to local area networks. In addition, cost of a laser which is the major part of a ONU is still relatively high.
It can be seen that there is a need for a cost-effective, but higher bandwidth in both downstream transmission and upstream transmission, network access system.
It can also be seen that there is a need for a low cost alternative to FTTC or SDV (Switched Digital Video) network access system while still retaining FTTC's basic advantages.